Simple edge loop won't Grid Fill
As stated by the Blender manual's section about the Grid fill command:


  Grid Fill uses a pair of connected edge-loops to fill in a grid that
  follows the surrounding geometry.




Despite this, using the vertices indices and the picking order, the operator is capable to guess the user intention even if he defines the whole boundary, but it still needs to guess the right offset's and span's values because you are not providing edgeloops but only some vertices and it has to guess which gruops of them belong to an edgeloop.



See in this sequence how the operator behaves differently on same geometry simply by picking another vertex to complete the selection.